In many polymerization processes for the production of normally solid polymer, a stream is formed which is a slurry of particular polymer solid suspended in a liquid medium, ordinarily the reaction diluent. Many of these polymerization processes employ a continuous reactor, such as a loop reactor, having a downwardly depending settling leg wherein polymer accumulates. In such polymerization processes the settling leg is periodically opened and the slurry contain therein is passed on to further processing steps wherein the polymer and diluent are separated. Typically the separation involves the passing of the slurry from the settling leg into a flash tank.
A typical example of such a process is disclosed in U.S. Pat. No. 4,424,341, the disclosure of which is incorporated herein by reference.
As noted in the afore mentioned U.S. Pat. No. 4,424,431, it as been known in the past to employ a flash line heater to supply vaporization heat to the slurry as it is transferred from the settling leg to the flash chamber.
The term flash line heater as used herein refers to an elongated conduit the interior of which is heated indirectly. Generally, most flash line heaters are double pipe heat exchangers. The reaction diluent is vaporized in the inner pipe utilizing the heat supplied from condensing steam in the annulus between the inner and outer pipes. The design of flash line heaters is more complicated than a simple heat exchanger because the flash line operates intermittently, co-transports a multiphase mixture (solid, liquid, and vapor), and the flow velocity in the line varies over a wide range. One problem in the design and use of flash line heaters is controlling the flow rate in the flash line independent of the polymer slurry discharge rate. Since the flash line should not limit the process flow they have been typically designed to have a flow capacity greatly in excess of what flows from the reactor. Typically only 5 to about 15% of the flow capacity has been used. When a flash line heater is designed in this fashion the low utilization of flow capacity results in a large heat transfer area that is used for a short period of time. An object of the present invention is to provide for a more effective use of the heat transfer capabilities of a flash line heater.
A further object of the present invention is to provide more compact flash line heater systems for polymer recovery.
Still another object of the present invention is to provide for more efficient use of the indirect heat exchange fluid, such as steam.
Other aspects, objects, and advantages of the present invention will be apparent from the following disclosure and FIG. 1.